The present invention relates in general to a color image receiving tube or picture tube of shadow mask type. More particularly, the invention is concerned with a faceplate panel of the picture tube having an improved structure.
Referring to FIG. 1 of the accompanying drawings, a color picture tube of shadow mask type is constituted by a glass envelope 4 including a rectangular faceplate panel 1, a tubular neck portion 2 and a funnel-like portion 3 for connecting together the faceplate panel 1 and the neck portion 2. On the other hand, the faceplate panel 1 is composed of a display faceplate la and an outer peripheral flange or side wall portion lb hermetically bonded to the funnel-like portion 3 by means of bonding glass having a low melting point as indicated by a reference numeral 5. A tricolor phosphor screen 6 is formed over the inner surface of the faceplate panel 1a.
A shadow mask 7 is mounted on the inner side of the faceplate panel 1 with a predetermined distance from the phosphor screen 6. An electron gun assembly 8 is mounted within the neck portion 2 in an in-line, triangular or delta array, wherein three electron beams 9 generated by the electron gun assembly 8 are directed toward the phosphor screen 6 through the shadow mask 7. A magnetic deflection yoke 10 is externally mounted in the vicinity of and around a junction 3 between the portion 2 and the funnel-like portion 3. By means of this yoke 10, magnetic fluxes are caused to act on the electron beams 9 in both horizontal and vertical directions, whereby the screen 6 is scanned with the electron beams 9 in the horizontal direction, i.e. along the major axis X--X and in the vertical direction, i.e. along the minor axis Y--Y so that a rectangular raster is generated on the screen 6.
Heretofore, the surface contour of the faceplate panel 1 has commonly been spherical or cylindrical. Attempts for realizing the panel surface as flat as possible has encountered various problems. First, difficulty arises in assuring a sufficient mechanical strength of the enclosure or tube. Additionally, in the shadow mask type color picture tube, there will occur a so-called doming phenomenon, that is, local dislocation or shift in color and hence deterioration in color purity. This is due to thermal expansion of the shadow mask 7 under irradiation with the electron beams 9. More specifically, when a given region of the shadow mask is heated to higher temperature than the other, a spherical bulging takes place in the given region, whereby the mask holes formed in that region are positionally displaced, as a result of which the relative position between the electron beams and the phosphor dots are correspondingly varied and thus the local color dislocation (color purity shift) is visually observed. This is the phenomenon referred to as "doming".
For having a better understanding of the invention, preparatory analysis will be made in some detail on the doming phenomenon by referring to FIGS. 2 to FIGS. 5A and 5B of the accompanying drawings, in which FIG. 2 shows in a front view the faceplate panel of the picture tube shown in FIG. 1, FIG. 3 is a fragmental sectional view of the picture tube taken along the line X--X in FIG. 2, FIG. 4 is an enlarged fragmental view of the faceplate and the shadow mask in a portion indicated as enclosed by a circle 12 in FIG. 3, and FIGS. 5A and 5B are enlarged fragmental views showing in section the screen in two different states, respectively. In the case of aspherical faceplate panel, the inner surface thereof presents a substantially spherical contour. In conformance with the spherical inner surface of the faceplate panel, the shadow mask assumes substantially a spherical curvature. As the surface profile or contour of the faceplate is caused to approximate to a flat plane, the spherical contour of the shadow mask becomes straightened approximately to a flat plane, which in turn involves angular deviation between the direction normal to a plane of the shadow mask and the direction in which the electron beam travels. In other words, the angle of incidence at which the electron beam lands the shadow mask becomes large. As the temperature of the shadow mask is increased under irradiation with the electron beam, the former is thermally expanded. As a consequence, the shadow mask is displaced in the direction normal to the plane of the shadow mask, as indicated by an arrow 14 in FIG. 4, from the solid line position 7 to a broken line position 7', as shown in FIG. 3. Correspondingly, the positions of the holes formed in the shadow mask are also displaced substantially in the direction normal to the shadow mask. At that time, an angular difference .alpha. makes appearance between the beam running direction 16 and the direction 14 in which the shadow mask is displaced, as is illustrated in FIG. 4. Consequently, the path 9 of the electron beam passing through a same hole in the shadow mask varies in such a manner as indicated by a broken line 9', in accompaniment to the thermal expansion of the shadow mask. This is visually observed as the dislocation of color (purity shift of color). More specifically, in the state in which no doming phenomenon takes place, the electron beam 9 can land on a center region between black matrix stripes 18, as shown in FIG. 5A, whereas it lands on at a position deviated from the center between the black matrix stripes, as indicated by 9' in FIG. 5B, upon occurrence of the doming phenomenon, giving rise to generation of the color dislocation.
Magnitude of change in the relative position between the electron beam and the phosphor dot as caused by the doming phenomenon, i.e. magnitude D of the doming can be calculated in accordance with the following expression (1): ##EQU1## where d represents a change in the hole position of the shadow mask in the direction normal thereto due to the thermal expansion of the mask, .alpha. represents the angle of incidence of the electron beam to the shadow mask, Pr represents a distance between the center of a deflection plane and the shadow mask as measured along the direction of beam path, and q.sub.r represents a distance between the shadow mask and the phosphor screen as measured along the beam path, as is illustrated in FIG. 3.
In case the curved surface of the shadow mask is of a simple spherical contour, the aforementioned incident angle .alpha. can be calculated in accordance with ##EQU2## where R represents the radius of curvature of the spherical surface of the shadow mask, and P.sub.o represents distance between the center of deflection and the center of the shadow mask on the major axis.
Taking as an example a 21V" (90.degree.) color picture tube known heretofore, the radius of curvature R is about 840 mm, and P.sub.o and P.sub.r are about 281.5 mm and about 306.7 mm, respectively, (as measured at a point on the shadow mask distanced from the center thereof by 150 mm). Accordingly, the angle .alpha. is about 18.8.degree..
When the radius of curvature R is increased to about 1680 mm in an attempt to flatten the spherically curved contour of the shadow mask in the color picture tube mentioned above, then P.sub.o =281.5 mm and P.sub.r =313.1 mm. Accordingly, .alpha.=23.5.degree..
Thus, when the faceplate panel is flattened (by doubling the radius of curvature) as described above, magnitude of the doming is increased by a factor of about 1.3, as calculated in accordance with the aforementioned expression (1) on the assumption that the change of the hole position in the shadow mask is constant. However, the results of computer-aided analysis based on the so-called finite element method show that magnitude of the doming is increased at least by a factor of 2 when the radius of curvature R is doubled. It has been found that the value resulting from the computer-aided analysis approximately coincides with the data obtained from the measurement conducted by the inventors for a prototype tube manufactured for this purpose.
As will be appreciated from the foregoing, limitation is imposed on the attempt for flattening the surface contour of the faceplate panel because of the doming phenomenon. To say in another way, diminishing in the radius of curvature of the shadow mask which is effective for remedying the doming is in contradiction to the flattening of the faceplate panel.
As the picture tube known heretofore in which attempt is made to make the flattening of the faceplate compatible with reduction of the doming phenomenon, there may be mentioned a one disclosed in GB No. 2136200A, GB No. 2136198A and GB No. 2147142A. In the case of this known cathode-ray tube, the surface contour of the faceplate panel along the minor axis is so realized as to be represented by a quadratic expression, while the curvature in the center portion of the faceplate panel along the minor axis is selected greater than the curvature along the major axis.
FIGS. 6, 7 and 8 of the accompanying drawings show sections of the known faceplate panel described above, which sections are taken along the major axis X--X, the major axis Y--Y and a diagonal W--W in FIG. 2. In these figures, P represents height of the peripheral wall portion of the panel. According to the teaching disclosed in the literature cited above, there is provided a region where the quadratic expression representing the curvature along the diagonal assumes minus sign. Namely, there are provided inflexion points 22 (see FIG. 8) with a view to flattening the corner surface regions of the faceplate.
The above faceplate panel however suffers problems mentioned below. First, reflection of ambient illumination on the faceplate panel surface presents a problem although it depends on the design of the curved surface contour of the faceplate. More specifically, because of the presence of the inflexion points in the corner regions of the faceplate panel, ambient light image reflected thereon undergoes distortion in the region covering the inflexion point. For example, ambient light image of a lattice pattern will be distorted in such a manner as illustrated in FIG. 9 upon being reflected on the faceplate panel, to discomfort to the viewer. As the area of the region where the quadratic equation representing the curvature along the diagonal assumes a minus sign (i.e. inflexion point covering region 22) is increased, the mechanical strength of the shadow mask is reduced and becomes more susceptible to thermal deformation. In view of the fact that there exists a correlation between the doming phenomenon and the contour of the boundary portion defining the effective picture area of the faceplate, difficulty will be encountered in remedying the doming phenomenon. In other words, when the effective picture area defining boundary portion (region covering the point 22 in FIG. 8) is flattened so that the faceplate may look flat, then the doming phenomenon is more likely to take place, to another problem.